It is widely believed that most communication systems will eventually include optical fiber transmission equipment for large portions thereof. In broadband landline communications, optical fibers already surpass coaxial cable in new system deployment because they provide extremely large bandwidth at favorable cost. Moreover, it may not be long before optical fibers extend all the way to the home--replacing both copper wires and coaxial cables. And, similar to electrical transmission paths, it is also important to properly terminate optical transmissions paths within the system. For example, one of the limitations encountered in optical fiber systems relates to noise arising from optical power reflections. Such reflections generally occur at any discontinuity in an optical transmission path, including the end of an optical fiber where a portion of the incident light is reflected back toward the source. Optical power reflected in this manner may reflect once again when it arrives at the source or other discontinuity in the system, adding an unwanted noise component to the signal. Moreover, optical power that is reflected back into a source can also corrupt the fundamental operation of the source, typically a laser. (See, for instance, R. Rao et al., Electronics Letters, Vol. 22 (14) pages 731-732 (1986). Accordingly, control of reflections is an important consideration in the design of optical fiber systems.
Low reflectance attenuators are known in the art and one is described in U.S. Pat. No. 5,082,345 in which an attenuator disc, made from polymethylmethacrylate (PMMA) plastic, is slidably suspended from a longitudinal slot in an alignment sleeve. Spring-loaded optical plugs are inserted into opposite ends of the sleeve and engage opposite sides of the disc to provide between 5 and 20 dB of attenuation depending on its thickness. However, such an arrangement is cumbersome and costly. Moreover, PMMA has an index of refraction of about 1.50 which, at best, produces -40 dB reflection, which is too much.
U.S. Pat. No. 5,263,103 discloses an optical fiber terminator comprising a length of core-less, non-guiding (undoped or doped) silica fiber having substantially the same diameter as the fiber to be terminated (e.g., 125 .mu.m). The amount of reflection varies according to the length and composition of the terminator, and by the use of non-reflecting coating applied to the optical fiber terminator. The optical fiber terminator is then attached to an active fiber by fusion splicing or index-matched epoxy. Unfortunately, handling such a small component is cumbersome, and conventional fusion splicers typically require that the fiber ends be relatively flat--meaning that this small component may need to be additionally processed prior to installation. Furthermore, in many applications, the optical fiber to be terminated is already installed in a standard optical connector, and it is undesirable to strip away the connector merely to access the optical fiber itself.
The last-mentioned difficulty is solved by another known terminator which comprises a length of fiber that is held within a cylindrical ferrule. The cylindrical ferrule includes an end face which abuts the optical fiber to be terminated. One end of the length of fiber terminates in the end face and is cleaved perpendicular (e.g., 90.degree.) to the central axis of the ferrule, whereas the other end of the length of fiber is cleaved at an angle (e.g., 80.degree.) with respect to the central axis. And while such a terminator provides good results, its manufacturing cost is far too high.
What is sought and what does not appear to be available in the prior art is an optical terminator which is inexpensive to manufacture and easily attaches to an optical fiber that is already installed within a cylindrical ferrule.